A Black opaque mould flux can be obtained in the reduction process of mould flux for continuous steel casting by silicon. This mould flux would enable mild cooling of steel since the flux can reduce both the radiative and conductive heat fluxes between the steel shell and the mould. For further utilisation of the mould flux without fluorine, the origin of the colour has been studied. Synthesized mould flux powder containing iron oxide was melted with an additive of silicon powder at 1673 K for 5 - 60 min followed by quenching to form glassy sample. Samples reduced for 5-30 min showed black opaque colour, while after 45-60 min reduction samples showed slightly bluish grey colour and showed transparency. Many iron-based particles were found in all the samples with micrometre order in diameter. The iron particles seemed to stuck together and to grow to form larger particles with increasing reduction time. Scattering efficiency of the particle was calculated to reveal the effect of iron particle dispersion on the optical properties of the mould flux. It was found that both scattering on the iron particle and absorption by iron efficiently increase the scattering efficiency in the mould flux.